When astronomical events are observed using a remote system such as a satellite, it is required to synchronize the time in the satellite system with that of the ground time in order to know the time of generation of observed events. With recent advance in the field of computers, high speed data processing has become possible in the satellite system, and also an advanced protocol such as packet telemetry based on recommendation by CCSDS (Consultative Committee for Space Data Systems) for communications between a satellite system and a ground station are used.
Because of the necessity of data processing in a satellite or employment of packet telemetry, ambiguous delay in time interval is generated from the time when data is acquired until the time when the data is transmitted to the ground. Therefore, it is difficult to estimate the time of generation of an event from a time when the data is received at the ground station. On the other hand, it is required to know the accurate time of generation of data while realizing a high speed data processing in the satellite system or an advanced protocol such as packet telemetry. Especially in astronomical observation, in order to verify the result of observation to that acquired by other satellites or that obtained on the ground, precision in time measurement of the order of microseconds is required.
In the conventional technology, observed data is sampled according to the timing generated by an apparatus for controlling the timing of operations of the entire satellite system. The data is inserted at a fixed location in the transfer frame and the time at which the data was generated is determined from the time when the transfer frame is received. On the other hand, a system in which request for the current time is made when required to a device which controls the system time, or a system in which standard time is determined by using data input time into a device generating a transfer frame is employed in a satellite system employing the packet telemetry therein.
In recent years, time is determined by using the GPS (Global Positioning System). FIG. 15 shows the commonly used GPS system. The system shown in FIG. 15 is a satellite system in which an orbiting satellite 105 acquires the signals, namely the observation data, from four GPS satellites 101 to 104. In this satellite system, the satellite 105 acquires apparent distances between itself and the GPS satellites 101 to 104 based on the acquired observation data, and obtains four unknown parameters i.e. its own position (x, y, z) and the difference between its own time and the time in the GPS satellites. With this method, an accurate time can be acquired in the satellite 105.
In the satellite system described above, in association with the advance in the computer technology, sophisticated processing such as data compression or data extraction has become possible, so that data length of the observed data or the like changes, and sometimes waste of resources occurs in data transfer when a fixed data format like that in the conventional technology is used. Efforts have been made in order to improve the efficiency in data transmission by employing packet telemetry such as CCSDS.
However, in the satellite system described above, because the complicated data processing such as the packet telemetry like CCSDS is employed, a delay is generated until the acquired data is packetized, or ambiguous delay is generated until a packet including data is actually edited into the transfer frame. Further, fluctuations in time from generation up to transfer of data to the ground station becomes larger, so that it is difficult to decide the time at which the data was generated from the time at which the data is received.
In a system in which time is required to be acquired by the a device which manages the time in the satellite, non-uniformity of around a couple of tens of microseconds is included in association with realization of a protocol for acquiring time, so that an error which is not desirable in a system requiring accurate time may be generated.
High precision time determination can be realized with GPS shown in FIG. 15 having been employed and becoming popular in recent years. However, the system configuration is very complicated. Further, in the example shown in FIG. 15, because the satellite 105 itself rotates around the Earth at a high speed a Doppler shift is generated. This Doppler shift makes the use of GPS on the ground difficult.